The present invention relates to an image forming apparatus configured to form an image by superimposingly transferring visual images carried by a plurality of image carriers onto a recording medium carried by a conveyer member or an intermediate transfer body.
An example of a conventional image forming apparatus is a so-called intermediate transfer tandem printer. In such an image forming apparatus, a plurality of image carriers carrying visual images (developing agent images) on the outer circumferential surfaces thereof are rotatably supported and spaced substantially evenly apart. A drive device rotates the image carriers in respective circumferential directions. A visual image forming device forms visual images on the outer circumferential surfaces of the respective image carriers, in synchronization with the rotation of the image carriers. A conveyer device keeps an intermediate transfer body in abutment with the image carriers while moving the intermediate transfer body at a speed in accordance with the moving speeds of the outer circumferential surfaces of the image carriers, so as to superimposingly transfer the visual images formed on the respective image carriers at abutting positions between the intermediate transfer body and the image carriers.
In the above image forming apparatus, periodic variations are caused in the moving speeds of the outer circumferential surfaces of the image carriers at the abutting positions between the respective image carriers and the intermediate transfer body (hereinafter referred to as “abutting position speeds”) due to variations in the outer configurations of the image carriers.
Specifically, when each of the image carriers has a drum-like configuration, variations in the roundness of the outer shape lead to differences in the distance from the rotation axis to the outer circumferential surface. Then, the moving speed of a portion distant from the rotation axis is faster than the moving speed of a portion close to the rotation axis even if the image carrier is rotated so as to have a constant rotation angle speed. Accordingly, the abutting position speeds vary periodically.
When there are periodic variations in the abutting position speeds in the image forming apparatus, specifically when the abutting position speeds of the image carriers become faster than the moving speed of the intermediate transfer body, an image deviation is caused, in which visual images transferred from the image carriers onto the intermediate transfer body are shortened In contrast, when the abutting position speeds of the image carriers become slower than the moving speed of the intermediate transfer body, an image deviation is caused, in which visual images transferred from the image carrier onto the intermediate transfer body are elongated.
As described above, the periodic variations in the abutting position speeds of the image carriers produce image deviations, thereby causing an adverse effect on an image formed by the image forming apparatus. Since variations in the outer configuration of the image carrier cannot be completely avoided, occurrence of periodic variations in the abutting position speeds of the image carriers should be accepted to some extent.
To reduce adverse effects of image deviations, an image forming apparatus, for example, disclosed in the Publication of Unexamined Japanese Patent Application No. 11-119502 has been provided. In the image forming apparatus, as shown in FIG. 10A, four photoconductor drums 3Y, 3M, 3C and 3Bk as image carriers are arranged to be spaced substantially evenly with respect to an intermediate transfer belt 5 (an intermediate transfer body). The intermediate transfer belt 5 is stretched between a drive roller 62 and a follower roller 60 in an endless manner and is moved in the right direction in the figure by the rotation of the drive roller 62. By rotating the photoconductor drums 3Y, 3M, 3C and 3Bk in synchronization with one another in a counterclockwise direction in the figure, toner images (visual images) carried by the photoconductor drums are superimposingly transferred onto the intermediate transfer belt 5 to thereby form a multicolor image. In this case, the photoconductor drums 3Y, 3M, 3C and 3Bk are arranged at a distance d, corresponding to the circumferential length c (=π×diameter B) of the photoconductor drums 3Y, 3M, 3C and 3Bk, apart from one another.
In the image forming apparatus as above, when the photoconductor drums 3Y, 3M, 3C and 3Bk are arranged such that the orientations of maximum points G, each of which is a position on the circumferential surface where the abutting position speed is the maximum, are in the same orientations, the operation of transferring the toner images from the photoconductor drums 3Y, 3M, 3C and 3Bk onto the intermediate transfer belt 5 is as described below.
To facilitate better understanding, it is assumed here that the abutting position speeds of the photoconductor drums 3Y, 3M, 3C and 3Bk vary in a sinusoidal manner due to eccentricity of the rotation axes of the photoconductor drums 3Y, 3M, 3C and 3Bk or other factors.
As shown in FIG. 10A, the photoconductor drums 3Y, 3M, 3C and 3Bk are in a state in which the maximum points G simultaneously abut the intermediate transfer belt 5 at a point in time (a timing T1). As time passes, the abutting position speeds of the photoconductor drums 3Y, 3M, 3C and 3Bk respectively vary in a sinusoidal manner as shown in FIG. 11A. FIGS. 10B through 10D show respective states of the photoconductor drums 3Y, 3M, 3C and 3Bk at timings T2 through T4, i.e. after each quarter rotation of the photoconductor drums 3Y, 3M, 3C and 3Bk from the timing T1. The orientations of the maximum points G of the photoconductor drums 3Y, 3M, 3C and 3Bk are shifted at the timing T2, T3 and T4 by 90°, 180° and 270°, respectively, with respect to the orientations at the timing T1. When the photoconductor drums 3Y, 3M, 3C and 3Bk are rotated by 360°, the respective maximum points G abut the intermediate transfer belt 5 again in the state as shown in FIG. 10A.
The intermediate transfer belt 5 is moved by the same distance as the circumferential length c of the photoconductor drums 3Y, 3M, 3C and 3Bk while the photoconductor drums 3Y, 3M, 3C and 3Bk are rotated by 360°. Accordingly, the phases of periodic variations in the abutting position speeds of the photoconductor drums 3Y, 3M, 3C and 3Bk and timings, at which the toner images on the photoconductor drums 3Y, 3M, 3C and 3Bk are transferred onto the intermediate transfer belt 5, are synchronized with each other.
Then, points on the intermediate transfer belt 5 at which transferred toner images are slightly shortened or elongated coincide with one another.
The above image forming apparatus can reduce deviation among visual images to be superimposed to thereby form a clear image since the phases of periodic variations in the abutting position speeds and the timings, at which the visual images on the image carriers are transferred onto the intermediate transfer belt, are synchronized with one another.